Light Sources Utilizing Segmented LEDs to Compensate for Manufacturing Variations in the Light Output of Individual Segmented LEDs

ABSTRACT

A light source and method for making the same are disclosed. The light source includes a plurality of Segmented LEDs connected in parallel to a power bus and a controller. The power bus accepts a variable number of Segmented LEDs. The controller receives AC power and provides a power signal on the power bus. Each Segmented LED is characterized by a driving voltage that is greater than 3 times the driving voltage of a conventional LED fabricated in the same material system as the Segmented LED. The number of Segmented LEDs in the light source is chosen to compensate for variations in the light output of individual Segmented LEDs introduced by the manufacturing process. In another aspect of the invention, the number of Segmented LEDs connected to the power bus can be altered after the light source is assembled.

BACKGROUND OF THE INVENTION

Light emitting diodes (LEDs) are an important class of solid-statedevices that convert electric energy to light. Improvements in thesedevices have resulted in their use in light fixtures designed to replaceconventional incandescent and fluorescent light sources. The LEDs havesignificantly longer lifetimes and, in some cases, significantly higherefficiency for converting electric energy to light.

For the purposes of this discussion, an LED can be viewed as havingthree layers, the active layer sandwiched between two other layers. Theactive layer emits light when holes and electrons from the outer layerscombine in the active layer. The holes and electrons are generated bypassing a current through the LED. The LED is powered through anelectrode that overlies the top layer and a contact that provides anelectrical connection to the bottom layer.

The cost of LEDs and the power conversion efficiency are importantfactors in determining the rate at which this new technology willreplace conventional light sources and be utilized in high powerapplications. The conversion efficiency of an LED is defined to be theratio of optical power emitted by the LED in the desired region of theoptical spectrum to the electrical power dissipated by the light source.The electrical power that is dissipated depends on the conversionefficiency of the LEDs and the power lost by the circuitry that convertsAC power to a DC source that can be used to directly power the LED dies.Electrical power that is not converted to light that leaves the LED isconverted to heat that raises the temperature of the LED. Heatdissipation often places a limit on the power level at which an LEDoperates. In addition, the conversion efficiency of the LED decreaseswith increasing current; hence, while increasing the light output of anLED by increasing the current increases the total light output, theelectrical conversion efficiency is decreased by this strategy. Inaddition, the lifetime of the LED is also decreased by operation at highcurrents.

Single LED light sources are not capable of generating sufficient lightto replace conventional light sources for many applications. In general,there is a limit to the light per unit area of LED that can bepractically generated at an acceptable power conversion efficiency. Thislimit is imposed by the power dissipation and the electrical conversionefficiency of the LED material system. Hence, to provide a higherintensity single LED source, larger area chips must be utilized;however, there is a limit to the size of a single LED chip that isimposed by the fabrication process used to make the LEDs. As the chipsize increases, the yield of chips decreases, and hence, the cost perLED increases faster than the increase in light output once the chipsize increases beyond a predetermined size.

Hence, for many applications, an LED-based light source must utilizemultiple LEDs to provide the desired light output. For example, toreplace a 100-watt incandescent bulb for use in conventional lightingapplications, approximately 25 LEDs having chips of the order of 1 mm2are required. This number can vary depending on the color temperaturedesired and the exact size of the chips.

In addition, the light source typically includes a power supply thatconverts either 115V or 240V AC power to a DC level compatible withdriving the LEDs. The conversion efficiency of this power supply, often80% or less in cost-competitive products, also contributes to theoverall power-to-light conversion efficiency of the light source. Toprovide the maximum power delivery efficiency, the output of the powersupply should be near the peak voltage of the AC power, and the currentthat must be delivered across the various conductors in the light sourceshould be minimized to avoid resistive losses in the conductors. Atypical GaN LED requires a drive voltage of about 3.2-3.6V. Hence, froma power conversion efficiency point of view, the 25 LED light sourcedescribed above would be constructed as a single string of 25 LEDsconnected in series with an output voltage level from the power supplyof approximately 80 volts.

However, there are other considerations such as the cost and reliabilityof the light source that must be taken into consideration in addition tothe power-to-light conversion efficiency. From a reliability point ofview, a single series connected string of LEDs is the poorest option. Ingeneral, LEDs are more likely to fail by forming an open circuit than ashort circuit. For example, a wire-bond that connects a pad in the LEDto external circuitry can fail. Hence, a single LED failure in theseries-connected string leads to the catastrophic failure of the lightsource.

From a reliability point of view, a light source in which all of theLEDs are connected in parallel would appear to be the best if thepredominant failure mechanism is LEDs failing by forming open circuits.If a single LED fails and a constant current source is used to drive theparallel connected LEDs, the current through the other LEDs willincrease slightly, and hence, the other LEDs will partially compensatefor the light lost when one of the LEDs fails. Unfortunately, such anarrangement is inefficient from the point of view of the power supplyefficiency and requires conductors that can handle very large currentswithout introducing significant transmission costs.

In addition to reliability and power conversion efficiency, the designermust provide a design that can accommodate the variations in the lightgeneration efficiency among individual LEDs. LEDs are fabricated onwafers that have some degree of non-uniformity across the wafer as wellas variations from wafer to wafer. As a result, the amount of lightgenerated by commercially available LEDs has a significant variationfrom LED to LED. The allowable variation in the light output of thefinal light source is determined by the need to have light sources thatall generate the same amount of light and have the same appearance. Ingeneral, the variation in light output among the LEDs is too great tomeet the needs of the light source manufacturers without some sorting ofthe LEDs to provide LEDs with less variability. The sorting process addsto the costs of the light source. In addition, many light sources cannotutilize LEDs that are not within a range of intensities that is lessthan the spread in intensities of the LEDs as manufactured. As a result,there is less of a market for LEDs that are not in the range ofinterest, which increases the cost of the LEDs in the desired range anddecreases the cost of the LEDs that are outside that range.

The problems inherent in balancing reliability against power supplyefficiency are reduced by constructing light sources in which aplurality of component light sources are connected in parallel. Eachcomponent light source consists of a plurality of LEDs connected inseries, and hence, utilizes a significantly higher driving voltage thanthe individual LEDs. For example, a typical GaN LED requires a drivevoltage of about 3.2 volts and a current of 0.35 amps. To provide alight source having approximately 2000 lumens, 25 such LEDs must bedriven. The light source can be constructed by connecting 5 componentlight sources in parallel. Each component light source consists of 5LEDs connected in series. Hence, the driving voltage is improved by afactor of 5 to 16V. If one LED fails by becoming an open circuit, theremaining 4 component light sources still function, and hence, the lightsource continues to function, albeit at a reduced brightness. However,the remaining LEDs are overdriven by 20 percent since these LEDs mustpass the current that no longer passes through the open circuitedcomponent light source. As a result, the lifetimes of the remaining LEDsis significantly shortened.

Unfortunately, this strategy does not eliminate the need for utilizingonly a subset of the production run of LEDs for any given final lightsource.

SUMMARY OF THE INVENTION

The present invention includes a light source and method for making thesame. The light source includes a plurality of Segmented LEDs connectedin parallel to a power bus and a controller. The power bus accepts avariable number of Segmented LEDs, the number being chosen to provide apredetermined light output for the light source. The controller receivesAC power and provides a power signal on the power bus. In one aspect ofthe invention, each Segmented LED is characterized by a driving voltagethat is greater than 3 times the driving voltage of a conventional LEDfabricated in the same material system as the Segmented LED. The numberof Segmented LEDs in the light source is chosen to compensate forvariations in the light output of individual Segmented LEDs introducedby the manufacturing process. In another aspect of the invention, thenumber of Segmented LEDs connected to the power bus can be altered afterthe light source is assembled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a light source according to thepresent invention.

FIG. 2 is a top view of Segmented LED 60.

FIG. 3 is a cross-sectional view of Segmented LED 60 through line 2-2shown in FIG. 2.

FIG. 4 is a top view of Segmented LED 70 showing the narrow p- andn-electrodes in adjacent segments that are connected by a set ofinterconnect electrodes.

FIG. 5 is a cross-sectional view of Segmented LED 70 through line 5-5shown in FIG. 4.

FIG. 6 is a top view of a segmented LED 75 that includes metalelectrodes that enhance the current spreading on the ITO layer FIG. 7illustrates one aspect of the present invention in a light sourceaccording to one embodiment of the present invention.

FIG. 8 illustrates a light source according to another embodiment of thepresent invention.

FIG. 9 illustrates an AC light source according to another embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The manner in which the present invention provides its advantages can bemore easily understood with reference to FIG. 1, which illustrates oneembodiment of a light source according to the present invention. Lightsource 20 includes a plurality of Segmented LEDs 21 that are connectedin parallel to a DC constant current source. The average current througheach Segmented LED is set by controller 22, which includes an AC to DCpower converter.

Segmented LEDs are discussed in detail in co-pending U.S. patentapplication Ser. No. 12/208,502, filed Sep. 11, 2008, which is herebyincorporated by reference. A detailed discussion of a Segmented LED willalso be provided below. For the purposes of the present discussion, itis sufficient to note that each Segmented LED is defined to be a singleLED die that is divided into N segments that are serially connected toone another, where N>1 and typically between 2 and 100. Each segment is,in effect, a small LED. The area of each Segmented LED is N timessmaller than the area of a conventional LED, and hence, each SegmentedLED generates substantially the same amount of light as a conventionalLED but with 1/N times the current i.e. at the same areal currentdensity. However, the driving voltage needed to run a segmented LED issubstantially equal to N times the voltage needed to run a conventionalLED in the same material system. Note therefore the segmented LEDproduces substantially the same amount of light with the same electricalpower input as a conventional LED chip of the same size, except thecurrent required is N times smaller but voltage required is N timeslarger. A segmented LED can be considered to be a replacement for aseries-connected string of N smaller LED chips, each of which beingsubstantially 1/N times the size of the conventional LED chip. TheSegmented LED, however, generates only 1/N of the light of a componentlight source composed of N series-connected conventional LEDs. As aresult, a light source that required 25 conventional LEDs connected as 5component light sources, consisting of 5 series-connected conventionalLEDs, connected in parallel, now requires 25 segmented LEDs connected inparallel. Hence, each segmented LED accounts for only 1/25^(th) of thelight output. As a result, the output of light source 20 can be finetuned by adding or removing individual segmented LEDs.

In contrast, the light output of the equivalent light source utilizingthe serial-connected strings of conventional LEDs cannot be easily tunedby adding or subtracting a single LED. Changing the number of LEDs inone of the serial connected component strings presents challenges, sinceadding or subtracting an LED changes the driving voltage of that string.Hence, a separate power source would be needed for each componentstring, which would increase the cost of the light source. To avoidthis, all of the component strings would need to be modified, and hence,5 LEDs would need to be added or removed. Similarly, adding or removingone entire series-connected string from the power rails changes thelight output by 5 LEDs or 20%. Hence, designs that utilize componentstrings consisting of N serially-connected strings of conventional LEDsare limited to adding or replacing N LEDs at a time. This limits thedegree to which a design can be tuned by changing the number ofconventional LEDs in the design.

In principle, the conventional light source design could utilizeconventional LEDs that are 1/N smaller in size so that adding orremoving N LEDs provides the same degree of tuning as the presentinvention. However, such a design has N times more LEDs, which increasesthe cost of manufacture.

The embodiments of the present invention described above depend on aSegmented LED. Refer now to FIGS. 2 and 3, which illustrate a segmentedLED light source that could be utilized with the present invention. FIG.2 is a top view of Segmented LED 60, and FIG. 3 is a cross-sectionalview of Segmented LED 60 through line 2-2 shown in FIG. 2. Segmented LED60 includes two segments 64 and 65; however, it will be apparent fromthe following discussion that light sources having many more segmentscan be constructed from the teachings of the present invention.Segmented LED 60 is constructed from the same three-layer LED structurein which the layers are grown on a sapphire substrate 51. The n-layer 52is grown on substrate 51, and then the active layer 55 and p-layer 53are grown over n-layer 52.

The segments 64 and 65 are separated by an isolation trench 66 thatextends through layer 52 to substrate 51 thereby electrically isolatingsegments 64 and 65. Isolation trench 66 includes a plateau 67 thatextends only partially into layer 52. The walls of isolation trench 66are covered by an insulating layer 57 that includes an open area 58 formaking electrical contact to the portion of layer 52 associated witheach segment. Insulating layer 57 can be constructed from any materialthat provides an insulating layer that is free of pinhole defects. Forexample, SiNx, SiOx, or other such dielectric films commonly used insemiconductor device fabrication can be used as the insulating material.Other materials can include polyimide, BCB, spin-on-glass and materialsthat are routinely used in the semiconductor industry for deviceplanarization.

Similar trenches are provided on the ends of Segmented LED 60 as shownat 68 and 69. A serial connection electrode 59 is deposited in isolationtrench 66 such that electrode 59 makes contact with layer 52 throughopening 58 in insulating layer 57. Electrode 59 also makes electricalcontact with ITO layer 56 in the adjacent segment. Hence, when power isprovided via electrodes 61 and 62, segments 64 and 65 are connected inseries. As a result, Segmented LED 60 operates at twice the voltage andhalf the current as a conventional LED.

In one aspect of the present invention, insulating layer 57 extendsunder electrodes 59 and 61 as shown at 57a in FIG. 3. Since electrode 59is opaque, electrode 59 blocks light generated in the portion of activelayer 55 immediately underlying electrode 59. In this regard, it shouldbe noted that the thickness of the layers shown in the figures is not toscale. In practice, the thickness of layer 53 is much smaller than thatof layer 52 and also much smaller than the typical width of electrodes59 or 61, and hence, electrode 59 blocks most of the light that isgenerated under electrode 59. Accordingly, current that passes throughlayer 55 under electrode 59 is substantially wasted, since most of thelight generated by that current is lost (absorbed by the opaque metal inmultiple reflections). The insulating layer extension blocks currentfrom flowing through this wasted area of layer 55, and hence, improvesthe overall efficiency of the light source. A similar issue is presentunder electrode 61, and hence, the insulating layer is extended underthat electrode as well.

In the embodiments shown in FIGS. 2 and 3, electrode 59 extends over theentire width of Segmented LED 60. As noted above, the portion of segment65 that underlies electrode 59 is non-productive and optically lossy,since light generated below electrode 59 is blocked and absorbed byelectrode 59. This leads to reduced light conversion efficiency as wellas reduced efficiency of utilization of the die surface, and hence,increased cost for the light source since additional active die areamust be provided to compensate for this lost area. Refer now to FIGS. 4and 5, which illustrate another embodiment of a segmented LED that canbe utilized with the present invention. FIG. 4 is a top view ofSegmented LED 70, and FIG. 5 is a cross-sectional view of Segmented LED70 through line 5-5 shown in FIG. 4. The cross-sectional view ofSegmented LED 70 through line 2′-2′ shown in FIG. 4 is substantially thesame as that shown in FIG. 2 with the exception of electrode 59 whichwould be replaced by electrode 78 and the narrow n- and p-electrodes 71and 72 shown in FIG. 4; hence, this cross-sectional view has beenomitted from the drawings.

Segmented LED 70 differs from Segmented LED 60 in that the wideinterconnect electrode 59 has been replaced by a plurality of serialelectrodes such as electrodes 78 and 79. These electrodes can be only5-10 microns wide and spaced approximately 150 microns apart, and thus,cover a much smaller area on segment 65 than electrode 59. Accordingly,the loss in efficiency discussed above is substantially reduced. Inaddition, the n-electrode 72 and p-electrode 71 have been replaced bynarrow electrodes that include wider pads 71′ and 72′ for wire bondingto external circuitry. In one preferred embodiment, the serialelectrodes are spaced apart by a distance that is more than 5 times thewidth of the electrodes so that the area covered by the serialelectrodes is significantly less than the width of the segments that arebeing connected in the Segmented LED.

The number of serial connection electrodes that are needed depends onthe conductivity of ITO layer 56. There must be sufficient serialconnection electrodes to assure that current is spread evenly over ITOlayer 56. The width of the serial connection electrodes is set by theamount of current that must be passed between segments, and hence,depends on the conductor used, the thickness of the conductor, and thenumber of serial connection electrodes. In the regions of segment 65that are not covered by a serial connection layer, the isolation trench77 does not require an insulating layer, and hence, the underlying LEDstructure receives power and generates useful light.

The distribution of current on ITO layer 56 can be improved byincorporating a number of narrow metal electrodes on the surface of ITOlayer 56. Refer now to FIG. 6, which is a top view of a segmented LED 75that includes metal electrodes that enhance the current spreading on theITO layer. Electrodes 73 are connected to the p-electrode 71 and to theserial connection electrodes between segments. The conductivity of themetal electrodes is much greater than that of the ITO layer, and hence,these very thin electrodes can provide increased current spreadingwithout blocking a significant fraction of the light generated in theunderlying structures. In one aspect of the invention, the area blockedby the electrodes is less than 20 percent of the light emitting area,and preferably less than 10 percent.

An optional thin n-electrode 73′ can be incorporated on the exposedn-layer to enhance the spreading of current on the n-layer. Thiselectrode does not block the generation of light, and hence, electrode73′ can cover a larger area than electrodes 73.

The serial connection electrodes can be constructed by depositing alayer of any suitable conductor over an insulating pad that is depositedin the isolation trench that separates the segments. Such an insulatingpad is shown at 171. It should be noted that the serial connectionelectrodes can be constructed from metals or ITO. ITO has the advantageof being more transparent, and hence, less light is blocked by theserial connection electrodes. However, ITO is more resistive, and hence,a larger area may be needed.

The Segmented LEDs require less drive current than conventional LEDsgenerating the same light, since the Segmented LEDs operate at highervoltages. As a result, a switch can be connected in series with aSegmented LED without substantially altering the efficiency of theSegmented LED due to power dissipation in the switch. Refer now to FIG.7, which illustrates this aspect of the present invention in a lightsource according to another embodiment of the present invention. Lightsource 80 is constructed from a plurality of Segmented LEDs 81 that arepowered from controller 82. Each Segmented LED is connected to one ofthe power rails by a switch 84 that is under the control of controller82. Controller 82 can utilize the switch to remove a Segmented LED thatfails by forming a short circuit. In addition, controller 82 canincrease or decrease the brightness of light source 80 by actuating oneor more of the switches. Although the switches are shown as mechanicalswitches, it is to be understood that any type of switch havingsufficiently low impedance in the conducting state could be utilizedincluding transistors or other semiconductor-based devices that can evenbe integrated with the controller as a single driver IC.

As noted above, one of the important advantages of utilizing SegmentedLEDs in the present invention is the ability to provide a light sourcethat operates from a significantly higher potential than conventionalLEDs while breaking up the light source into sufficient component lightsources to compensate for the variability in light generation betweenthe various component light sources.

Consider a light source that is constructed from individual LEDs thathave a manufacturing variation characterized by G. As noted above, thisvariation arises from variations across individual wafers on which theLEDs are fabricated and on wafer-to-wafer variations in any givenproduction run. For example, the variation in light output from LED toLED could be approximated by a Gaussian distribution having a standarddeviation σ, although the precise form of the distribution of LEDoutputs is not critical in the following discussion.

In general, the final light source must meet some design specificationrelated to the variation in light output from light source to lightsource and the uniformity of light intensity within each light source.The design tolerance assures that the light sources are interchangeable.That is, the light outputs of different manufactured light sources areindistinguishable in appearance and intensity to an observer of thelight from the light sources.

Refer now to FIG. 8, which illustrates a light source 90 according toanother embodiment of the present invention. Consider the case in whichM component light sources 91 are connected in parallel between two powerrails 92 and 93 that can accommodate a variable number of light sources.M is chosen such that, on average, M component light sources generatethe desired light output as specified in the design specification of thelight source. Assume that the average light output of each componentlight source is less than the variation in the light output specified bythe design tolerance. In this case, one could populate the light sourcewith M component light sources and measure the light output. If theoutput is too high, one or more component light sources are removed tobring the output of the light source within the design specification. Ifthe output is too low, one or more component light sources are added tothe light source.

In the prior art, the component light sources are series connectedstrings of conventional LEDs so that each component light source can beoperated at a potential that can be efficiently provided. Consider alight source that is designed to provide 2000 lumens of light (about theequivalent of a 100 watt incandescent light). Conventional LEDs thatgenerate 80 lumens provide a good compromise between yield and die size.Hence, 25 such LEDs, or 5 series connected strings are utilized betweenthe power rails. Hence, each component light source generates about 20percent of the target light output, and the removal or addition of onesuch string would result in a 20 percent change in the light output ofthe light source, which would exceed the typical design tolerancesallowed for such light sources. Hence, the LEDs must be matched withineach series string to assure that the light output of each series stringhas a small enough variance to assure that precisely 5 strings willprovide the target output to within the design tolerance. The cost ofthis type of binning and matching is significant for inexpensive lightsources.

Alternatively, controller 94 could be programmed for each light sourceto provide more or less current to the power rails, and hence,compensate for the variation in the outputs of the individual LEDs. Thisprocedure requires measuring the light output of each light source andvarying the current to provide the desired result. The controller mustinclude a variable output current source and storage for a parameterthat specifies the correct current. Such controllers increase the costover the simple controller required by a light source that utilizes 25Segmented LEDs and is adjusted by adding or subtracting Segmented LEDsfrom the power rails.

The arrangement shown in FIG. 8 can also be used to construct an AC LEDlight source in which the individual component light sources are powereddirectly from an AC power source that has been rectified by a full-waverectifier bridge to provide essentially DC voltage that can be appliedacross each component light source composed of M series-connectedstrings of N-segment segmented LEDs such that the total number M*N ofLED junctions times the typical operating voltage Vf of a conventionalLED equals approximately the peak voltage V_(pk) (equal to square-rootof two times the rms voltage) of the rectified-AC voltage. Typically,M*N*V_(f) is chosen slightly less than V_(pk) in order to overdrive theLEDs some of the time to compensate for the time when the LEDs are notturned ON when the voltage is less than their turn-ON voltage.Alternatively, a large capacitor (generally needs to be of theelectrolytic type to provide the high capacitance at high voltagenecessary for such applications) can be connected in parallel to turn onthe LEDs during those “OFF times” by discharging through them. Thisconcept is well known but involves the use of costly and low life-timecapacitors which increases cost and reduces the lifetime of the lightsource. Refer now to FIG. 9, which illustrates an AC light sourceaccording to one embodiment of the present invention. Light source 100is similar to light source 90 discussed above with reference to FIG. 8in that a variable number of component light sources are connectedbetween power rails 92 and 93, the number of light sources being chosento provide the desired light output to within a design tolerance. Lightsource 100 differs from light source 90 in that controller 104 providesan AC output voltage between the power rails and half the componentlight sources, e.g. 101, 102 are connected across the power rails in theopposite orientation. On each half cycle of the AC power signal, half ofthe component light sources will generate light and the other half willbe off. This method does require twice as many LEDs but saves the costand complexity of adding rectifier diodes.

In one aspect of the present invention, controller 104 includes atransformer having a fixed primary to secondary ratio. Such embodimentsare particularly attractive from a cost point of view. However, theoutput voltage signal applied to the power rails is subject to thevariability of the transformer manufacturing process. By varying thenumber of component light sources that are connected across the powerrails at the time of manufacture, the light source can be adjusted toprovide a standard output independent of the variations in controller104, thus providing a lower cost light source than sources that requirea variable output controller to compensate for manufacturing variations.

The above described light sources depend on adjusting the number ofcomponent light sources connected to the power rails at the time ofmanufacture to compensate for differences in the light output of theindividual component light sources or the output of the controller thatpowers the component light sources. In one aspect of the presentinvention, the number of component light sources is adjusted by startingwith a light source that has M component light sources, where M ischosen such that the light source will have either the correct number orone or two additional component light sources that are not needed toprovide the design light output. The light source is tested afterassembly. The light source will either have the correct light output oran output that is too high. If the output is too high, a link 106 in oneof the power rails is interrupted by laser ablation or a similar processto prevent one of the component light sources from being powered duringnormal operation. The light source could have multiple links of thistype to allow more than one component light source to be removed.

This method provides an assembly process that can be completelyautomated at the cost of including one or two additional component lightsources. If the component light sources are Segmented LEDs, the cost isessentially the same as that of an additional LED die or two, which isacceptable in many applications.

It should be noted that some binning of the LEDs, even in the case ofSegmented LEDs, is required to provide uniformity over the light source.The physical size of a light source with multiple LEDs is sufficientlylarge to allow a user to detect local non-uniformities arising from LEDsthat have sufficiently different light output. In general, the variationin light intensities from manufacturing variations is greater than themaximum variability allowed within a light source. Such variations inlight output occur across a wafer or from wafer to wafer on a productionrun. Accordingly, the LEDs must be sorted into bins having similar lightoutput to assure that all of the LEDs in any given bin are sufficientlyuniform when viewed by an observer.

Denote the maximum allowable variation of the Segmented LEDs across alight source by σ_(m). That is, the Segmented LEDs with a light sourcemust have an intensity within the range of intensities defined byI±σ_(m). In general, the manufacturing variation is characterized byσ>σ_(m). Hence, the Segmented LEDs are divided into non-overlappinggroups (“bins”) in which each group is characterized by a mean lightintensity. Consider a process in which there are two such groupscharacterized by mean intensities of I₁ and I₂, respectively. Theminimum number of Segmented LEDs used to fabricate a light source ischosen such that, MI₁ is equal to (M+1)I₂ to within the designspecification for the light source. Typically, the variation within anybin is approximately 10 percent.

For example, consider the case in which a 2000 lumen light source is tobe constructed from Segmented LEDs that are nominally 80 lumens perSegmented LED. Suppose the Segmented LEDs vary from 70 lumens perSegmented LED to 90 lumens per Segmented LED across a lot of wafers. TheSegmented LEDs are binned into 4 bins, the first being Segmented LEDshaving outputs from 70 to 75 lumens, the second being Segmented LEDshaving outputs from 75 to 80 lumens, the third bin being Segmented LEDshaving outputs from 80 to 85 lumens, and the fourth bin being SegmentedLEDs having outputs from 85 to 90 lumens. The target light source can beconstructed by utilizing 29 Segmented LEDs from the first bin, or 26Segmented LEDs from the second bin, or 23 Segmented LEDs from the thirdbin, or 22 Segmented LEDs from the fourth bin. Since the power railsaccommodate a variable number of Segmented LEDs, each choice for thelight source can utilize the same circuit carrier and layout andessentially all of the Segmented LEDs from the manufacturing run can beutilized. In addition, the light sources do not need to be measured andtrimmed after production to meet the design specification. However, suchmeasurement and trimming could be utilized to further reduce thevariability from final light source to light source.

As noted above, the preferred component light source is a Segmented LED.In general, to provide efficient delivery of power to the componentlight sources that are connected in parallel, the component lightsources must operate at a voltage that is substantially greater thanthat of a single junction LED, which is typically 3.2 volts forGaN-based LEDs. In one embodiment of the present invention, a SegmentedLED with 5 segments is utilized to provide a component light source witha 16V drive voltage. However, component light sources with a drivevoltage of at least 3 times that of a single junction LED in thematerial system in question can be advantageously utilized.

The above-described embodiments of the present invention have beenprovided to illustrate various aspects of the invention. However, it isto be understood that different aspects of the present invention thatare shown in different specific embodiments can be combined to provideother embodiments of the present invention. In addition, variousmodifications to the present invention will become apparent from theforegoing description and accompanying drawings. Accordingly, thepresent invention is to be limited solely by the scope of the followingclaims.

1. A light source comprising: a plurality of Segmented LEDs connected inparallel to a power bus, said bus accepting a variable number ofSegmented LEDs, said number being chosen to provide a predeterminedlight output for said light source; and a controller that receives ACpower and provides a power signal on said power bus.
 2. The light sourceof claim 1 wherein each Segmented LED is characterized by a drivingvoltage that is greater than 3 times the driving voltage of aconventional LED fabricated in the same material system as saidSegmented LED.
 3. The light source of claim 1 wherein each Segmented LEDgenerates light at an intensity that is within a predetermined intensityrange that is less than 10% of the average light intensity generated bysaid Segmented LEDs.
 4. The light source of claim 1 wherein said powerbus includes a breakable link that disconnects one of said SegmentedLEDs from said power bus.
 5. The light source of claim 4 wherein saidbreakable link comprises a region of conductor that is removable bylaser ablation.
 6. The light source of claim 1 wherein said power busincludes a switch that disconnects one of said Segmented LEDs inresponse to a command from said controller.
 7. A method for fabricatinga light source, said method comprising: providing a substrate havingfirst and second power rails; providing a controller that generates apower signal on said first and second power rails; connecting MSegmented LEDs in parallel between said first and second power rails,each Segmented LED generating an intensity of light within a firstpredetermined intensity range when powered by said power signal.
 8. Themethod of claim 7 further comprising measuring a light output from saidlight source and altering the number of Segmented LEDs connected to saidpower rails if said measured light output differs from a target lightoutput.
 9. The method of claim 8 wherein one of said Segmented LEDs isdisconnected from one of said power rails.
 10. The method of claim 9wherein said one of said Segmented LEDs is disconnected by breaking alink in one of said power rails.
 11. (canceled)
 12. A light sourcecomprising: a substrate; a light emitting structure comprising: a firstlayer of semiconductor material of a first conductivity type depositedon said substrate; an active layer overlying said first layer; and asecond layer of semiconductor material of an opposite conductivity typefrom said first conductivity type overlying said active layer; a barrierthat divides said light emitting structure into first and secondsegments that are electrically isolated from one another; a serialconnection electrode that connects said first layer in said firstsegment to said second layer in said second segment; a first powercontact electrically connected to said second layer in said firstsegment; and a second power contact electrically connected to said firstlayer in said second segment, wherein said first and second segmentsgenerate light when a potential difference is created between said firstand second power contacts, wherein said serial connection electrodecomprises a plurality of isolated conductors that span said barrier. 13.The light source of claim 12 wherein said second layer comprises ap-type semiconductor and wherein said light source further comprises atransparent electrode overlying said second layer and a plurality ofcurrent spreading electrodes extending over said transparent electrodefor distributing current to different portions of said transparentelectrode, said current spreading electrodes being electricallyconnected to said serial connection electrode.
 14. The light source ofclaim 13 wherein said current spreading electrodes cover less than 20percent of said second layer.
 15. The light source of claim 12 whereinsaid plurality of isolated conductors is characterized by a conductorwidth in a direction perpendicular to said barrier and wherein saidisolated conductors are spaced apart from one another by a distancegreater than 5 times said conductor width.